How can you simplify class hierarchy design?

Abstraction is not related to hiding details. That is a common misconception, but encapsulation hides details. Abstraction doesn't even have them. About the examples, though, using Animals as examples of classes is overly abused and simplistic, and leads to a lot of confusion. It is better to focus on more computer related things, like characters in a game.

Abstraction should be like a blueprint. It is not there to hide anything but to provide basic minimal structure without that this class doesn't make sense. abstract class should represent a blueprint, such that derived classes can use it to implement and provide the real world implementation of it.

In the realm of class hierarchy in object-oriented programming, classes are organized in a hierarchy based on substitutability. This means that if one class can be substituted for another in a given context, they are part of a hierarchy. For example, consider two classes, X and Y. If X can be substituted by Y, then X is considered the parent or ancestor of Y within the class hierarchy. This relationship is also known as "is a" relationship and is also expressed as "Y is a subclass of X" and "Y inherits from X." This concept is fundamental to understanding how classes relate to each other in object-oriented programming.

In general when it comes to Abstraction you actually want to be as general as possible, but as specific as is useful but no more. The example you chose is a perfect example of why. It is very difficult to conceive of a reason as to why breed should be the differentiator in a class hierarchy rather than a member of a class either as a string or an enumeration. Meanwhile I can conceive of many domains in which differentiating the type of animal (dog, cat, bird, etc) may solve some useful concern. In general if the only difference between classes is the values on a class rather than the definition of the class itself then it shouldn't be a separate class, because it's simply not a useful abstraction.

Abstraction is the disregard of unnecessary details. For example, the software class Animal may be unconcerned with the color of the animal, so color is omitted from the attribute set. In a different domain, color might be such an important item that it is represented by a Color class, which might have RGB and CYMK as the realized subclasses. It is up to you as the developer to understand the domain and which pieces are important enough to retain, and which are insignificant enough to be abstracted away.

This paragraph needs to be rewritten. It talks about LSP but does not talk about the derived class method should not expect more and should not do less contract. The example itself is abstract. Needs to be detailed.

LSP is not about simplification, it is about logical consistency. It simply means that the designer should not change the meaning of an inherited operation in a way that would be confusing to other users of the hierarchy, or that would change the fundamental meaning for the program if the base was used in place of the subclass. For example if the base class creates an .add(int x) method which performs addition to a numeric attribute; no subclass should override that with an .add(int x) method which converts x to a string and appends it to a string attribute. That would violate LSP.

So... use the most complicated principle you can find to simplify your code? Sounds "good"... but can you give an actual example that works? Liskov Substitution Principle is not about simplifying the code, but making it correct. If subclasses have stricter constraints than their parent classes, they cannot be used in place of their parents or else the code would break. I would argue that there's nothing to simplify if something like that happens... there's only code to fix.

Does this really simplify the class hierarchy design or is it simply the good practice of upholding the contract of the interface you're implementing? The answer is it's the latter. This entire paragraph needs to either be deleted or rewritten, because while an important principle it has nothing to do with the thesis of the paragraph or the article which is "how do I simplify a class hierarchy".

Choose a good metaphor. When a design looks complex but does not seem to be able to be reduced, a good metaphor leverages the understanding people already know (similar to a well know design pattern) and has the effect of reducing the cognitive load of learning the design even though, technically - the complexity may be unchanged.

"Composition over inheritance" doesn't belong to class hierarchy design because there is no "hierarchy" in composition; it's in parallel. I would move this part to the "What else to consider" section, to state that inheritance based OOD pattern is good for traditional "waterfall" projects that have dedicated business requirements and take year to built, while composition is a more agile approach that is good to adapt ever-changing business requirements, which is more and more common in modem world.

Composition .vs. inheritance is a tradeoff that can be considered, but you're trading complexity in the hierarchy with complexity in the class. There can be benefits, in that a "has-a" relationship can be changed at runtime, whereas "is-a" cannot. However, without inheritance, there is also no polymorphism, eliminating one of OO's major benefits. For these reasons, and others, the developer should consider carefully where it makes sense to use composition over inheritance, and not just apply the principle blindly.

"instead of creating a class named Car that inherits from a class named Vehicle, you might want to create a class named Car that has a component named Engine". Sure about the usage of "instead of"? Both can be valid depending upon the context of the problem

You're not really expressing the point of "Composition over Inheritance". Let's go back to your earlier example of dog breeds: adding the field "breed":String, rather than creating a german shepherd, husky, tamaskan, golden retriever, ..., etc individualized breed classes allows you to have one Dog class rather than hundreds of dog breed classes. That is the fundamental point of: Composition over inheritance. Further to this point in proper OOP inheritance is a tool of last resort to the point that modern OOP languages such as Rust have completely tossed out inheritance based polymorphism in favour of interface based polymorphism.

Your premise is missing Types: Object oriented design is about a Typed language, declarative or imperative. Usually the latter. Types are what are being composed, not classes. Classes speak of the abuse of the "is a" relationship by making a Cat a Dog in order to get access to some of the dog's methods. Types declare a "is a" relationship when they follow the Liskov-substition Principle (See above). "Composition over inheritance" refers to the idea of composing "is a" types via interfaces, rather than using the relationship for code reuse (inheritance via Extends).

Before starting refactoring, it might be worth it to have a look at the Test Driven Development method. When using this development method you get in a rhythm where refactoring is part of the process. Having extensive test coverage also gives you confidence to refactor fearlessly because all intended functionality is validated on each test run.

Refactoring is a software development technique that involves modifying the structure of existing code without changing its functionality. It can help you simplify complex class hierarchies, improve code readability, and reduce technical debt. But refactoring is not something you should do randomly or regularly without a clear purpose. To make the most of refactoring, you need to have specific goals, prioritize tasks, plan and communicate effectively, test rigorously, and document changes. By following these steps, you can use refactoring as a tool to enhance your code and solve underlying problems, rather than creating unnecessary code churn.

Definitely believe in refactor your code often. The code overall will constantly evolve and the sooner we refactor, the lesser tech debt will be created. However, 1 thing i still think is we should not refactor too much if the product is not going to 1. Drastically change and you are giving just some fixes. 2. You dont have much active developers on that code.

You need a section about Interfaces over Inheritance. Interface based polymorphism is THE standard today and language designers have pretty much agreed on this point ever since the invention of Java which is why nobody but C allows for Multiple Inheritance, meanwhile they'll allow you to implement as many Interfaces as your heart desires, and some schools of thought even call for using Interfaces as "Type Tags" (a usually empty interface that serves as a flag that a class has some particular functionality). This is especially effective in languages like C# and Rust which allow you to separate the definition of the data of the class from it's functionality through partial classes.

Several object oriented programming languages support generics - an important aspect of object oriented programming, allowing classes and interfaces to be defined with placeholder types. One of the advantages of this is better extensibility by means of co-variance and contra-variance.

This article give some vague gotcha's about object oriented design, but nothing a first year IT bachelor student wouldn't learn in the first few weeks. It needs a lot more depth to be an interesting read.

Simplifying class hierarchy design is partly programming language dependent. Rust for instance, doesn't even support class inheritance. So, any complexity of class hierarchy design is avoided to begin with, statically, by the language. Class hierarchies in C , which supports inheritance of implementation or of interface, can be simplified by avoiding implementation inheritance and avoiding "protected" member data. Public class derivation can also be avoided entirely in C . This can be done sometimes using static polymorphism through template code. Or always using something increasingly called polymorphic *value* types.

As about business logic (BL), first of all, a very deep understanding of entities and relationships is a must. That leads automatically, to the simplification of the class hierarchy (BL). The same is, and for the UI (on the part connected to the BL).